The present invention relates to the casting of molten metals, particularly continuous casting apparatus wherein at least one of the mold surfaces is an endless metal band and a method of improving the useful life of the endless band.
Continuous molten metal casting apparatus of the wheel-band type, or alternatively of the twin (or multiple) belt type, wherein an elongated endless band forms one of the mold surfaces, such as by partially enclosing a groove inscribed about the periphery of a rotatable casting wheel are well known and have been overwhelmingly adopted for commercial production of non-ferrous metals in recent decades. With higher-temperature metals such as steel, band life is significantly reduced by heating and erosion effects. Band parting, especially sudden and unexpected band breaking during casting, is or can be calamitous. To avoid this problem bands have either been replaced at regular intervals long before band deterioration or expensive and relatively complex mechanical systems have been adopted to overcome short band life.
The present invention avoids these and other problems and additionally provides a method of drying coolant remaining on the belt from the casting operation.
The apparatus is of a configuration such that it may be used or retrofitted with a great many of the wheel-band and twin-belt casting machines and casting machine configurations of past and present manufacture.
In one typical system for continuously casting a molten substance, an endless flexible metal band is guided about one or more generally fixed position idler or tension band wheels and a portion of the peripheral groove of a casting wheel rotatable about a fixed axis. For the purpose of example but not limitation, the casting of molten metals such as copper, steel, and aluminum, or alloys thereof, will be discussed. The molten metal to be cast is poured in a continuous manner into the moving mold portion of the casting apparatus formed by the band and the peripheral groove in the casting wheel as the casting wheel is rotated by an external drive mechanism. Coolant is applied to the external and/or internal surfaces of the wheel and to the outside surface of that portion of the band which closes the peripheral groove of the casting wheel, said coolant acting functionally to extract the heat from the molten metal at a rapid rate and to prevent the casting wheel and band from overheating. At the time the metal band is removed from the peripheral groove of the casting wheel by the band guide wheels the previously molten metal is sufficiently solidified to permit extraction from the casting wheel and be guided on to a succeeding stage in the continuous casting process.
In one such casting apparatus, using an 84" diameter wheel casting a 4 square inch bar section, the prior art band life extends typically to a range of 8 to 10 hours, or approximately one work shift.
A key component in the operation of such wheel-band molten metal casting apparatus is the band. Indeed, economics created by the wheel-band machines in the past which have reduced the cost of the manufactured rod/strip/wire product while increasing product quality are largely attributable to the inherent continuousness of operation of such machines. Once wheel life is maximized well beyond band life, band life becomes a factor most limiting the continuousness of operation of the casting machine. Extending band life therefore functions to reduce operating down time, maintenance time, and operating and maintenance costs. Other systemic advantages accompany extended band life, including longer component lifetimes for thermally cycled system parts (pour pots, launders, pour spouts, casting wheels, furnaces, and burners), more energy efficient casting, greater overall productivity, reduced scrap product, longer life of subsequent mill rolls (due to fewer startups), and a better quality product.
In the operation of wheel-band continuous casting systems of the type generally described above, one of the major problems is the care, maintenance, and replacement of the band. Due to the need to form the band into arcs to pass around the casting wheel and band guide wheels, it must be made of flexible materials. Additionally, thinner bands permit more efficient transfer of heat from the molten metal than thick bands while having two drawbacks; first, thin bands are more subject to band tensioning difficulties, and second, thin bands undergo more strenuous thermal cycling, compounding any tension difficulty tendencies. The two most common failure modes are due to thermal and machanical stresses; both effects must be carefully considered when selecting materials for casting bands. Other factors to be considered include band cost, cost of preparation, ease of installation, band life, safety, and heat extraction efficiency, the latter being of special import in the casting of alloys containing elements of differing solidification temperatures.
Generally, bands for this type of casting machine have been selected from among very low carbon steel alloys and copper and copper alloys when casting molten metals. One low carbon steel alloy in common use is A.I.S.I. 1006 or 1008 grade, having a good tensile strength (40,000 to 60,000 psi), low linear expansion, easily joined ends (TIG welding proves durable), is low in cost and in characterized by numerous other advantages when used for casting bands.
The selection of given band materials is, lastly, dependent on many other factors than life and wheel-compatible linear expansion factors. While the use of copper or copper alloy wheels is well known in the wheel-band continuous casting of copper, aluminum, steel, and other metals, the preferability of a band material with the characteristics of copper or copper alloys may be limited to the casting of only one or two metals or a few metal alloys. The accumulated experience in the art indicates that, for example, low carbon steel alloys are most suited to the widest range of metals and metal alloys, and is thus a more universal band material than copper or copper alloy bands. Low carbon steel bands have been found to be especially useful in the present invention.
Previous casting systems have incorporated pressurized air and heating or burner devices for drying the band and/or application of soot to the band surfaces. The present invention extends far beyond such devices in that such prior art devices did not increase band life while the present invention does so in a demonstratable manner, while also accomplishing the prior art object of band drying in a significantly improved manner. The previous band drying heaters were open, inefficient devices.
It is therefore obvious that there is a need in this art for certain improvements in operating economy such as may be obtained with greater operating lifetimes from inexpensive bands and, with increasing costs in the manufacture of metal casting machinery, reduced casting machine operating expense. Furthermore, a need exists for an improved, simplified band life extending apparatus intended for use with elongated bands, especially a band life extending apparatus which might at low cost be retrofitted to many prior art or previously manufactured casting machines.